1. Field of the Invention
This invention relates to light emitting diodes and more particularly to new structures for enhancing their light extraction.
2. Description of the Related Art
Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light and commonly comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. The light generated by the active region emits in all directions and light escapes the semiconductor chip through all exposed surfaces. Packaging of the LED is commonly used to direct the escaping light into a desired output emission profile.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved. New LEDs are being made from materials such as InAlGaN, which allows for efficient illumination in the ultraviolet to amber spectrum. Many of the new LEDs are more efficient at converting electrical energy to light compared to conventional lights and they can be more reliable. As LEDs improve, they are expected to replace conventional lights in many applications such as traffic signals, outdoor and indoor displays, automobile headlights and taillights, conventional indoor lighting, etc.
However, the efficiency of conventional LEDs is limited by their inability to emit all of the light that is generated by their active layer. When an LED is energized, light emitting from its active layer (in all directions) reaches the emitting surfaces at many different angles. Typical semiconductor materials have a high index of refraction (n≈2.2-3.8) compared to ambient air (n=1.0) or encapsulating epoxy (n≈1.5). According to Snell""s law, light traveling from a region having a high index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Light that reaches the surface beyond the critical angle will not cross but will experience total internal reflection (TIR). In the case of an LED, the TIR light can continue to be reflected within the LED until it is absorbed. Because of this phenomenon, much of the light generated by conventional LEDs does not emit, degrading its efficiency.
One method of reducing the percentage of TIR light is to create light scattering centers in the form of random texturing on the LED""s surface. [Shnitzer, et al., xe2x80x9c30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodesxe2x80x9d, Applied Physics Letters 63, Pgs. 2174-2176 (1993)]. The random texturing is patterned into the surface by using sub micron diameter polystyrene spheres on the LED surface as a mask during reactive ion etching. The textured surface has features on the order of the wavelength of light that refract and reflect light in a manner not predicted by Snell""s law due to random interference effects. This approach has been shown to improve emission efficiency by 9 to 30%.
One disadvantage of surface texturing is that it can prevent effective current spreading in LEDs which have a poor electrical conductivity for the textured electrode layer, such as for p-type GaN. In smaller devices or devices with good electrical conductivity, current from the p and n-type layer contacts will spread throughout the respective layers. With larger devices or devices made from materials having poor electrical conductivity, the current cannot spread from the contacts throughout the layer. As a result, part of the active layer will not experience the current and will not emit light. To create uniform current injection across the diode area, a spreading layer of conductive material can be deposited on the surface. However, this spreading layer often needs to be optically transparent so that light can transmit through the layer. When a random surface structure is introduced on the LED surface, an effectively thin and optically transparent current spreader cannot easily be deposited.
Another method of increasing light extraction from an LED is to include a periodic patterning of the emitting surface or internal interfaces which redirects the light from its internally trapped angle to defined modes determined by the shape and period of the surface. See U.S. Pat. No. 5,779,924 to Krames et at. This technique is a special case of a randomly textured surface in which the interference effect is no longer random and the surface couples light into particular modes or directions. One disadvantage of this approach is that the structure can be difficult to manufacture because the surface shape and pattern must be uniform and very small, on the order of a single wavelength of the LED""s light. This pattern can also present difficulties in depositing an optically transparent current spreading layer as described above.
An increase in light extraction has also been realized by shaping the LED""s emitting surface into a hemisphere with an emitting layer at the center. While this structure increases the amount of emitted light, its fabrication is difficult. U.S. Pat. No. 3,954,534 to Scifres and Burnham discloses a method of forming an array of LEDs with a respective hemisphere above each of the LEDs. The hemispheres are formed in a substrate and a diode array is grown over them. The diode and lens structure is then etched away from the substrate. One disadvantage of this method is that it is limited to formation of the structures at the substrate interface, and the lift off of the structure from the substrate results in increased manufacturing costs. Also, each hemisphere has an emitting layer directly above it, which requires precise manufacturing.
U.S. Pat. No. 5,793,062 discloses a structure for enhancing light extraction from an LED by including optically non-absorbing layers to redirect light away from absorbing regions such as contacts, and also to redirect light toward the LED""s surface. One disadvantage of this structure is that the non-absorbing layers require the formation of undercut strait angle layers, which can be difficult to manufacture in many material systems.
Another way to enhance light extraction is to couple photons into surface plasmon modes within a thin film metallic layer on the LED""s emitting surface, which are emitted back into radiated modes. [Knock et al., Strongly Directional Emission From AlGaAs/GaAs Light Emitting Diodes, Applied Physics Letter 57, Pgs. 2327-2329 (1990)]. These structures rely on the coupling of photons emitted from the semiconductor into surface plasmons in the metallic layer, which are further coupled into photons that are finally extracted. One disadvantage of this device is that it is difficult to manufacture because the periodic structure is a one-dimensional ruled grating with shallow groove depths ( less than 0.1 xcexcm). Also, the overall external quantum efficiencies are low (1.4-1.5%), likely due to inefficiencies of photon to surface plasmon and surface plasmon-to-ambient photon conversion mechanisms. This structure also presents the same difficulties with a current spreading layer, as described above.
Light extraction can also be improved by angling the LED chip""s side surfaces to create an inverted truncated pyramid. The angled surfaces provide the TIR light trapped in the substrate material with an emitting surface [Krames, et. al., High Power Truncated Inverted Pyramid (AlxGa1-x)0.5In0.5P/GaP Light Emitting Diodes Exhibiting  greater than 50% External Qauntum Efficiency, Applied Physics Letters 75 (1999)]. Using this approach external quantum efficiency has been shown to increase by 35% to 50% for the InGaAlP material system. This approach works for devices in which a significant amount of light is trapped in the substrate. For GaN devices grown on sapphire substrates, much of the light is trapped in the GaN film so that angling the LED chip""s side surfaces will not provide the desired enhancement.
Still another approach for enhancing light extraction is photon recycling [Shnitzer, et al., xe2x80x9cUltrahigh Spontaneous Emission Quantum Efficiency, 99.7% Internally and 72% Externally, From AlGaAs/GaAs/AlGaAs Double Heterostructuresxe2x80x9d, Applied Physics Letters 62, Pgs. 131-133 (1993)]. This method relies on LEDs having a high efficiency active layer that readily converts electrons and holes to light and vice versa. TIR light reflects off the LED""s surface and strikes the active layer, where it is converted back to an electron-hole pair. Because of the high efficiency of the active layer, the electron-hole pair will almost immediately be reconverted to light that is again emitted in random directions. A percentage of the recycled light will strike one of the LEDs emitting surfaces within the critical angle and escape. Light that is reflected back to the active layer goes through the same process again.
One disadvantage of this approach is that it can only be used in LEDs made from materials that have extremely low optical loss and cannot be used in LEDs having an absorbing current spreading layer on the surface.
The present invention provides new LEDs having light extraction structures that are disposed on an exposed surface or within the LED to increase the probability of light escaping from the LED; thereby increasing the LED""s light extraction and overall efficiency. The new LED is easy to manufacture and provides numerous new options and combinations for extracting light.
The new LED generally comprises an LED structure having a p-type layer, an n-type layer, and an active layer between the p-type and n-type layers. The LED structure is sandwiched between a first spreader layer and a second spreader layer. The spreader layers are semiconducting or conducting layers that distribute current across the plane of the device so that current is efficiently injected into the active layer. Light extraction structures are included that are on or within the new LED (or substrate). The structures provide a spatially varying index of refraction and provides surfaces to allow light trapped within the LED to refract or reflect and escape. In most embodiments the LED structure and current spreading layers are grown on a substrate that is adjacent to the first spreader layer, opposite the LED structure. Respective contacts are included on the first and second spreader layers and a bias applied across the contacts causes the LED structure""s active layer to emit light. The light extraction structures are preferably disposed in a plane parallel to the LED""s layers and substantially cover the area of the LED
The light extraction structures are preferably either arrays of light extraction elements (LEEs) or disperser layers. In those embodiments having an LEE array on an exposed surface, the array is formed from a material that has a higher index of refraction than the LED""s encapsulating material. The LEEs can be shaped using many different methods and provide many different surfaces for otherwise trapped light to escape.
Alternatively, the new LED can have the LEE arrays placed within the LED itself. The internal LEE arrays are also formed to provide a spatially varying index of refraction. The LEE array is formed during the LED growth process and once the array is formed the remaining layers of the LED structure are grown over the array by an epitaxial deposition technique to embed the LEE array within the LED. Light rays that would otherwise be trapped in the epitaxial layers or substrate can interact with the LEE array to refract and/or reflect into rays that can escape the LED.
Another embodiment of the new LED includes a disperser layer on one of the LED""s exposed surfaces, with the layer formed of a material having a higher index of refraction than the LED encapsulating material. Light that hits the disperser layer on the LED has an increased chance of being scattered into an escaping direction. By using a surface material to form the light disperser layer the problems of patterning roughness into the semiconductor surface are eliminated, providing an advantage over the work of Schnitzer.
Alternatively, the new LED can have disperser layers disposed within the LED itself. The disperser layer can be formed in or on the substrate prior to epitaxial growth of the LED, or within the LED epitaxial structure itself. The disperser layer is made from a material with an index of refraction that is different from the substrate and/or epitaxial material so that light scattering can occur.
Most of the above embodiments can also be mounted using flip-chip mounting techniques, with the substrate becoming the LEDs primary emitting surface.
These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: